To form the individual cylinders of the internal combustion engine, at least one cylinder head is connected at an assembly end side to a cylinder block. To hold the pistons or the cylinder liners, the cylinder block, which at least jointly forms the crankcase, has a corresponding number of cylinder bores. The pistons are guided in the cylinder liners in an axially movable fashion and form, together with the cylinder liners and the cylinder head, the combustion chambers of the internal combustion engine.
Internal combustion engines are ever more commonly being supercharged, wherein supercharging is primarily a method of increasing power, in which the air for the combustion process in the engine is compressed. The economical significance of said engines for the automobile industry is ever increasing.
In general, for supercharging, use is made of an exhaust-gas turbocharger in which a compressor and a turbine are arranged on the same shaft, with the hot exhaust-gas flow being supplied to the turbine and expanding in said turbine with a release of energy, as a result of which the shaft, which is mounted in a bearing housing, is set in rotation. The energy supplied by the exhaust-gas flow to the turbine and ultimately to the shaft is used for driving the compressor which is likewise arranged on the shaft. The compressor conveys and compresses the charge air supplied to it, as a result of which supercharging of the cylinders is obtained.
The advantage of the exhaust-gas turbocharger for example in relation to a mechanical charger is that no mechanical connection for transmitting power is required between the charger and internal combustion engine. While a mechanical charger extracts the energy required for driving it entirely from the internal combustion engine, and thereby reduces the output power and consequently adversely affects the efficiency, the exhaust-gas turbocharger utilizes the exhaust-gas energy of the hot exhaust gases.
Supercharged internal combustion engines are commonly equipped with a charge-air cooling arrangement by which the compressed combustion air is cooled before it enters the cylinders. In this way, the density of the supplied charge air is increased further. In this way, the cooling likewise contributes to a compression and effective charging of the combustion chambers, that is to say to an improved volumetric efficiency.
Supercharging is suitable for increasing the power of an internal combustion engine while maintaining an unchanged swept volume, or for reducing the swept volume while maintaining the same power. In any case, supercharging leads to an increase in volumetric power output and an improved power-to-weight ratio. For the same vehicle boundary conditions, it is thus possible to shift the load collective toward higher loads, where the specific fuel consumption is lower. This is also referred to as downsizing.
Problems are encountered in the configuration of the exhaust-gas turbocharging, wherein it is basically sought to obtain a noticeable performance increase in all rotational speed ranges. A severe torque drop is commonly observed in the event of a certain rotational speed being undershot. It has previously been sought to improve the torque characteristic of a supercharged internal combustion engine by various measures, for example by a small design of the turbine cross section and simultaneous exhaust-gas blow-off. If the exhaust-gas mass flow exceeds a critical value, a part of the exhaust-gas flow is, within the course of the exhaust-gas blow-off, conducted via a bypass line past the so-called wastegate turbine. Said approach however has disadvantages at relatively high rotational speeds.
The torque characteristic of a supercharged internal combustion engine may also be improved by virtue of a plurality of chargers—exhaust-gas turbochargers and/or mechanical chargers—being provided in the exhaust-gas discharge system in a parallel and/or series arrangement.
A supercharged internal combustion engine is thermally more highly loaded, owing to the increased mean pressure, than a conventional naturally aspirated engine, and therefore also places increased demands on the cooling arrangement. To keep the thermal loading within limits, a supercharged internal combustion engine is generally equipped with a cooling arrangement, hereinafter also referred to as engine cooling arrangement. It is fundamentally possible for the cooling arrangement to take the form of an air-cooling arrangement or a liquid-cooling arrangement. Since significantly greater amounts of heat can be dissipated by a liquid-cooling arrangement, an internal combustion engine of the present type is generally provided with a liquid-cooling arrangement. The internal combustion engine according to the disclosure is also a liquid-cooled internal combustion engine.
Liquid cooling requires that the internal combustion engine, that is to say the at least one cylinder head and/or the cylinder block, be equipped with a coolant jacket, that is to say requires the provision of coolant ducts which conduct the coolant through the cylinder head or block, which entails a complex structure. Here, the mechanically and thermally highly loaded cylinder head or block is firstly weakened in terms of its strength as a result of the provision of the coolant ducts. Secondly, the heat need not firstly be conducted to the surface to be dissipated, as is the case with the air-cooling arrangement. The heat is dissipated to the coolant, generally water provided with additives, already in the interior of the cylinder head or block. Here, the coolant is conveyed, such that it circulates, by means of a pump which is arranged in the cooling circuit and which is generally mechanically driven by means of a traction mechanism drive. The heat dissipated to the coolant is discharged from the interior of the cylinder head or block in this way, and is extracted from the coolant again in a heat exchanger. A ventilation vessel provided in the cooling circuit serves for ventilating the coolant or the circuit.
Like the internal combustion engine itself, the turbine of the at least one exhaust-gas turbocharger is likewise thermally highly loaded. As a result, the turbine housing according to the previous systems is produced from heat-resistant, often nickel-containing material, or equipped with a liquid-cooling arrangement in order to be able to use less heat-resistant materials. EP 1 384 857 A2 and the German laid-open specification DE 10 2008 011 257 A1 describe liquid-cooled turbines and turbine housings.
The hot exhaust gas of the supercharged internal combustion engine also leads to high thermal loading of the bearing housing and consequently of the bearing of the charger shaft. Associated with this is the introduction of a correspondingly large amount of heat into the oil which is supplied to the bearing for the purpose of lubrication. On account of the high rotational speed of the charger shaft, the bearing is formed generally not as a rolling bearing but rather as a plain bearing. As a result of the relative movement between the shaft and the bearing housing, a hydrodynamic lubricating film, which is capable of supporting loads, forms between the shaft and the bearing bore.
The oil should not exceed a maximum admissible temperature, because the viscosity decreases with increasing temperature, and the friction characteristics are impaired when a certain temperature is exceeded. Too high an oil temperature also accelerates the aging of the oil, wherein the lubricating characteristics of the oil are also impaired. Both of these phenomena shorten the service intervals for oil changes and can pose a risk to the functional capability of the bearing, wherein even irreversible destruction of the bearing and therefore of the turbocharger is possible.
For the above reasons, the bearing housing of a turbocharger of an internal combustion engine is frequently equipped with a liquid-cooling arrangement. Here, a distinction is made between the liquid-cooling arrangement of the bearing housing and the abovementioned liquid-cooling arrangement of the turbine housing. Nevertheless, the two liquid-cooling arrangements may—if appropriate only intermittently—be connected to one another, that is to say communicate with one another.
In contrast to the engine cooling or cooling of the turbine housing, the cooling of the bearing housing may be maintained even when the vehicle has been shut down, that is to say the internal combustion engine has been switched off, at least for a certain period of time after the internal combustion engine has been switched off in order to prevent irreversible damage as a result of thermal overloading.
This may basically be realized by an additional, electrically operated pump to which electricity is supplied for example by the on-board battery, which pump conveys coolant via a connecting line through the bearing housing when the internal combustion engine has been switched off and thereby ensures cooling of the bearing housing and of the bearing even when the internal combustion engine is not in operation. The provision of an additional pump is however a relatively expensive measure.
Also known are concepts which dispense with an additional pump. Here, a rising line is laid through the bearing housing of the exhaust-gas turbocharger, which rising line functions as a connecting line and leads through the bearing housing from the cooling circuit of the engine cooling arrangement to the ventilation vessel. The conveying of the coolant when the internal combustion engine is switched off is realized by the so-called thermosiphon effect, which is based substantially on two mechanisms.
Owing to the introduction of heat—which continues even when the internal combustion engine is switched off—from the heated bearing housing into the coolant situated in the rising line, the coolant temperature increases, as a result of which the density of the coolant decreases and the volume taken up by the coolant increases. Superheating of the coolant may furthermore lead to a partial evaporation of coolant, such that coolant passes into the gaseous phase. In both cases, the coolant takes up a larger volume, as a result of which ultimately further coolant is displaced, that is to say conveyed, in the direction of the ventilation vessel.
The formation of the cooling arrangement of the bearing housing using a rising line and utilizing the thermosiphon effect however does not lead to a supply of coolant to the bearing housing according to demand, which yields disadvantages.
Without further measures, coolant will be conveyed via the rising line through the bearing housing into the ventilation vessel even during the warm-up phase after a cold start, even though cooling of the bearing is not required at this time. The undesired conveying of coolant also opposes the desired fast warm-up of the assemblies to a minimum temperature or operating temperature.
Furthermore, the coolant throughput through the ventilation vessel should be as low as possible in particular at low coolant temperatures. The throughput should advantageously be completely prevented for as long as the coolant has not exceeded a predefinable minimum temperature. Firstly, a degassing process, that is to say a ventilation process, requires that the coolant is in the ventilation vessel for a certain residence time, for which reason the throughput should fundamentally be limited. Secondly, a low temperature of the coolant, or the higher viscosity of the coolant on account of the low temperature, has the effect that the coolant is enriched with air again as it flows out of the ventilation vessel—contrary to the actual objective. The latter is a basic problem with ventilation by ventilation vessels, but is particularly pronounced at low coolant temperatures, whereas toward higher temperatures, the re-enrichment of the coolant with air does not take place or can be disregarded. The coolant throughput likewise has an—albeit secondary—influence on the re-enrichment of the coolant with air, wherein an increasing throughput intensifies the effect.
The inventors herein have recognized the issues with the above approach and provide a supercharged liquid-cooled internal combustion engine to at least partly address them. In one example, a supercharged liquid-cooled internal combustion engine comprises a cylinder head connected at an assembly end side to a cylinder block. The engine also includes a cooling circuit including a pump for conveying coolant, a heat exchanger, and a ventilation vessel, and an exhaust-gas turbocharger including a compressor and a turbine arranged on a shaft which is rotatably mounted in a liquid-cooled bearing housing. The bearing housing is connected into the cooling circuit by a connecting line and arranged between the pump and the ventilation vessel. A valve is controlled as a function of coolant temperature arranged in the connecting line between the pump and the ventilation vessel.
According to the disclosure, the conveying of coolant via the connecting line through the bearing housing is prevented or minimized by a valve at low coolant temperatures, in particular during the warm-up phase after a cold start of the internal combustion engine. Together with the undesired conveying of coolant at low coolant temperatures, the problem, which arises in particular at said temperatures, of the re-enrichment of the coolant with air as it exits the ventilation vessel is also eliminated.
As a valve, use may be made of a self-controlled valve which, as a function of the coolant temperature, varies the flow cross section of the connecting line and thereby controls the coolant throughput through the bearing housing, in such a way that the throughput is increased with rising coolant temperature. Consequently, in the internal combustion engine according to the disclosure, not only is the undesired conveying of coolant at low temperatures counteracted, but rather also the conveying of coolant and therefore the cooling action is accelerated, that is to say increased, toward high temperatures by an increase in the throughput, that is to say by an opening of the valve. This results in a supply of coolant to the bearing housing according to demand, wherein the conveying of the coolant is based on the thermosiphon effect.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.